Organic light-emitting diodes, so-called OLEDs, have a forward current flowing through them during operation in the forward direction and exhibit electroluminescence phenomena in the process. In this case, the intensity of the electroluminescence is dependent on the magnitude of the forward current.
OLEDs usually have the disadvantage that ageing occurs, in the course of which the intensity of the electroluminescence decreases for the same forward current. Said ageing is accompanied by an increase in the forward resistance of the OLED. Corresponding behaviour is exhibited by a forward voltage dropped across the OLED given the same current. With the current flow remaining the same, said forward voltage rises with advancing ageing of the OLED. To put it in more general terms, the characteristic curve of an OLED is altered with advancing ageing. The ageing of the OLED can be regarded as a state which can bring about the same state of ageing independently of the type of current that flowed previously. In this case, a short high current flow leads to the same state as a long low current flow. There is the same situation concerning the behaviour of the image contents displayed on a display. A pixel which has been driven very bright for a short time attains the same state as a pixel which has been operated with low brightness for a long time. Therefore, the driving can be corrected correspondingly given knowledge of the ageing state.
A display can be formed from many OLEDs which have an individual ageing behaviour depending on the information represented.
The document US 2004/0070558 A1 describes an OLED display comprising OLED pixels which are controlled by means of a control circuit. The display comprises an OLED reference pixel, the voltage drop of which is determined by means of a measuring circuit. The measuring circuit is connected to an evaluation circuit, which generates a feedback signal as a reaction to the behaviour of the reference pixel. The feedback signal is fed to the control circuit in order that the latter can compensate for changes in the behaviour of the OLED pixels.
The document EP 1 318 499 A2 discloses an OLED display having a current source for generating a reference current and a driver transistor for controlling the OLED pixels. In one embodiment, the current source generates a current in a manner dependent on a luminosity setting signal of the display in order to set the total luminous intensity of the display.
The document US 2003/0122813 A1 discloses a method for controlling an OLED display. The method involves applying voltages for driving OLED pixels of the display. In order to compensate for the change in luminosity of the individual OLED pixels, the OLED pixels are driven individually and the current flowing through them is measured for each pixel and stored. Afterwards, the voltages present at the OLED pixels are controlled in accordance with the stored current values.
The document US 2003/0146888 A1 describes an OLED display that can be operated in two different modes. In a first mode, the OLED display is operated by means of a constant voltage, while in the second mode a constant current is used for this purpose.
The document DE 100 09 204 A1 describes a method for driving actively addressed OLED displays in which the current-voltage characteristic curves of the pixels are measured. The data of the current-voltage characteristic curve are written to a memory. If the current-voltage characteristic curves deviate from the ideal characteristic curve, then the image information stored in the image memory is correspondingly manipulated in order that the same brightness appears on the display despite the ageing of individual pixels.
For measuring the current-voltage characteristic curves, the column drivers of the display matrix are provided with measuring devices. The hardware outlay of the arrangement increases considerably in this case. The way in which the current-voltage characteristic curves of the pixels are measured is not described and is not obvious to the person skilled in the art.